The present invention relates to electronic apparatus, methods and computer program products for GPS applications and, more particularly, to apparatus, methods and program products for maintaining GPS navigational data.
Many portable electronic devices, such as cellular mobile terminals, personal digital assistants (PDAs), laptop computers, and the like, are now equipped with Global Positioning System (GPS) receivers. GPS is a space-based radio triangulation system using a constellation of satellites in orbit around the earth. A GPS receiver receives radio signals from various ones of the satellites and determines the times that the radio signals take to travel from the GPS satellites to the GPS receiver. By multiplying the travel time by the speed of light, the GPS receiver can calculate a range for each satellite in view. Ephemeris information provided in the satellite radio signal typically describes the satellite's orbit and velocity, thereby enabling the GPS receiver to calculate its position through a process of triangulation.
Determining the position of a GPS receiver typically requires the acquisition of a set of navigational parameters from the navigational data signals of four or more GPS satellites. This process may often take several minutes, as the duration of the GPS positioning process is generally dependent upon how much information a GPS receiver has initially. Most GPS receivers are programmed with almanac data, which coarsely describes the expected satellite positions for up to one year ahead. However, if the GPS receiver does not have some knowledge of its own approximate location, then the GPS receiver may not be able to find or acquire signals from the visible satellites quickly enough, and, therefore, cannot calculate its position quickly. Furthermore, it should be noted that greater signal strength is typically needed for capturing the C/A Code and the navigation data at start-up than is needed for continued monitoring of an already-acquired signal. It should also be noted that the process of monitoring the GPS signal may be significantly affected by environmental factors. Thus, a GPS signal, which may be easily acquired in the open, typically becomes harder to acquire when a receiver is under foliage, in a vehicle or in a building.
In order to improve GPS receiver performance, techniques have been developed to quickly provide GPS receivers with assistance data, e.g., time and position estimates, satellite ephemeris and clock information, and visible satellite list (which generally varies with the location of the mobile station), which can enable a GPS receiver to expedite position determination. Such assistance data may be transmitted, for example, from a terrestrial wireless communications system, e.g., a cellular wireless network or a wireless local area network (WLAN) access point. Such assistance data may not always be available, however, and a user may experience a significant increase in response time of the GPS receiver when such assistance data is not available, for example, when the user is traveling in an area not covered by the user's cellular service provider.